This invention relates to a marine drive which reduces cavitation of the propeller and permits a more shallow draft boat construction, and, more particularly, to a shroud and flanking rudders for use with power boats.
There have been literally hundreds of designs of boats attempting to improve the efficiency of boat propulsion and steering systems. Among these designs have been various tunnel hull configurations such as those described in U.S. Pat. Nos. 3,626,894; 2,896,565; or 3,793,980. While these tunnel designs have met with some degree of success they unfortunately offer a relatively expensive form of construction for the boat as well as occupy an undesired amount of space within the boat itself.
Other configurations have used out drives in which the motor drive shaft powers a propeller through a submerged gear box. This configuration permits good steering control but has the disadvantage of having a high degree of drag and requires the presence of a complex, expensive, sealed gear box, which itself is subject to leakage.
Often, the line of thrust provided by a propeller does not coincide with the center of hull resistance. This in itself can cause or produce unstable riding characteristics of the boat and, too, a portion of the forward thrust of the boat is lost, the loss being proportional to the cosine of the angle by which the line of thrust differs from the direction of motion.
Among all of these problems, however, cavitation of the propeller is perhaps one of the most troublesome. Cavitation is that effect produced by a propeller, particularly if it is not entirely submerged, to develop air pockets about the blades so that the blade has no solid surface of fluid to push against. At low speeds particularly, there is insufficient forward motion of the boat to permit a following propeller blade to meet solid water. Instead it tends to follow in the path of the preceding blade only to face an air pocket or cavity produced by the preceding blade. At higher speed air tends to be sucked into the propeller from the surface.
A planing boat acts as a displacement vessel. At low speeds, the horsepower required to drive the boat at a given speed is approximately a function of the cube of the boat velocity. With the higher angle of attack, under these conditions, i.e., the angle with which the boat approaches the surface of the water, the propeller is normally fully immersed. However, even with this full immersion cavitation can develop thereby reducing the effective thrust of the propeller. As the boat increases in speed, a point is reached at which the planing effect takes over and the dynamic effect of the water on the hull tends to lift the boat out of the water to assume a lower attack angle with greatly decreased drag. At this point, the horsepower required to drive the boat is reduced and normally varies as a function of the square of the boat velocity. With the boat now higher in the water, a real problem develops with cavitation of the propeller if a shallow draft of the boat is to be maintained for reduced drag, the propeller must be relatively close to the surface of the water without cavitating. Being close to the surface cavitation occurs all the more easily.
Accordingly, it is an object of this invention to provide an improved shroud for a boat propeller which reduces cavitation of the propeller.
A further object of this invention is to provide an improved shroud for a boat propeller which reduces cavitation of the propeller even when the propeller is close to the surface.
Many of the rudders designed for boats are of a type which are positioned immediately aft of the propeller itself so as to be in the helical wash of the propeller. This often creates a problem in that the turbulent flow from the propeller produces a much higher degree of drag on the rudder than would otherwise be desirable. To alleviate this problem, so-called flanking rudders have been designed. Unfortunately, most of the known type flanking rudders such as described in U.S. Pat. Nos. 3,710,794 and 3,872,817 are of a type which have an appreciable number of appendages underwater. These appendages do little more than to produce vastly increased drag. Further, if care is not taken, negative drag can occur which if unchecked can force the rudder into a full turn position.
Accordingly, it is a further object of this invention to provide an integrated drive-steering system with improved flanking type rudders which have minimum drag and facilitate effective steering.